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Welding, hydrotesting a compressor tank

01-24-2009, 08:23 PM

I purchased a compressor recently. It is a Speedaire, 80 gallon compressor with 7.5 HP motor and 80 gallon tank.

Based upon first impressions, the motor and pump run fine. However, when bringing the tank up to pressure, I heard a hissing noise at about 140 PSI, and realized that there is a hairline crack in the tank, near a 2" long weld attaching the engine/pump mounting platform to the tank.

The crack does NOT track the heat affected zone of the original weld, in fact it is perpendicular to it.

\====
\
\

Here the "=" signs denote the HAZ, and "\" signs denote the crack.

In any case, when I saw this happen, I almost dirtied my pants and immediately relieved the compressor of pressure.

I am not yet sure what was the cause of this crack in the first place. I will try to find what I can.

My question, obviously, concerns my repair options. I do weld and have some experience with both TIG, as well as with 7018.

But, I can hardly think of any welding where more is at stake than here, due to pressure. So, I see the following options:

1) Take off motor and pump, cut holes in the tank and throw it away, look for another tank.

2) Repair the tank by welding and hydrotest.

Considering option 2, the first question concerns welding. How would you weld? How do you identify where the crack ends? Would you drill relief holes at ends of the crack?

The second question is about hydrotesting. I was thinking about something simple, such as replace tha gauge with a 400 PSI gauge, close off all openings besides one, fill tank with water, connect to a hydraulic pump or grease gun, and bring pressure to 400 PSI (the tank supports 200 PSI) and look for leaks.

This is very time consuming and I would like to know how likely would it be that I would make some very bad mistake.

This comes up quite often, and my answer is always the same: Unless the tank is something really special (and this isn't), scrap it. It isn't worth the risk fixing it. New tanks are cheap. The crack you are describing is due to fatigue from vibration and the stress concentration at the mount point (quite common, by the way). Likely to show up elsewhere as well in the near future, and my show with undesirable manifestations. The tank is worn out. There is no practical (ie: less expensive than a new tank) way to detect and repair these flaws before they open up for most people.

I would suggest a replacement tank. Make up a frame to support the compressor rather than mounting to the new tank. Much better life that way, if you keep the tank drained of moisture. DO NOT weld mounts for the compressor to the new tank. If you must mount the pump directly on the tank, find a tank manufactured for the purpose, with the loading considered in the design.

As a side note, in most places in the US and Canada, it is not legal to repair the tank yourself. It must be done by an authorized shop (Natn'l board "R" stamp holder, or equivalent). Non-commercial users can get away with it, but if there is a failure, your insurance probably won't cover any damage or injury.

Comment

I'd choose option #1
I don't weld for a living so I'm by no means an expert, but I know several people who do weld for a living (my Dad mainly - Ok so he retired a few years ago at 71yrs young) and I can't think of one who would try to repair it. I do know some farmers and so called weldors who have tried repairing tanks, one of which spent 3 days in the hospital after a portable tank litterally got wrapped around his head (you could see where his nose, chin, and forehead are in the tank). The tank busted loose at about 90psi. I would have to beleive that if it is cracked in one spot, there's another one coming. I'm sure it could be fixed by someone that is willing to take a chance, but I hope like heck that no one is by it when it decides to let loose. I've seen the outcome of that, and trust me it aint pretty. Good 'ole flathead (his nickname even before the 'accident') kept that old tank as a stark reminder to the power of compressed air. Me, being the good friend and in general a smart A$$, bought him a new air tank after getting home from the hospital. That was probably 7 years ago - he still won't fill it.

For what it's worth - If you decide to scrap it, That is a good idea to cut holes in it so someone doesnt see it at the scrap yard, curb, or whatever and decide to try using it....

Comment

As others have said "dont fix it"
Dont cut holes in it!!!
Cut in in half long ways.
Make a grill or smoker.
A parts washer, Large turkey fryer.
But dont scrape it use it for a project.
Lay the 2 halves a their side and a la penutbutter sandwich, a beer tub.
The idears are coming fast. LOL

Be safe
Jeff

Give more than you get and
you will get more than ya need.
This is true for the good and bad
that life puts out.

Comment

If it were mine, I would break it down, (motor and Compressor), cut the motor base off the tank (I'd use a cut-off wheel) leaving about 1/4" on the tank to weld it back on. Inspect the tank for more cracks, buff off the paint from the crack area, degrease it (using a non flammable degreaser) V out the crack slightly, preheat the base metal (90-150 degrees) Propane torch is fine, Tig or Mig the crack, clean-up the welded area, Fill it with a water hose (hydro test) most water systems are around 60-80psi. If all is good, weld the base back on the tank and put it back in service. You wouldn't believe the cracks I've fixed in the area Refineries and the pressure and temperature they operate at.

Comment

Fill it with a water hose (hydro test) most water systems are around 60-80psi. If all is good, weld the base back on the tank and put it back in service. You wouldn't believe the cracks I've fixed in the area Refineries and the pressure and temperature they operate at.

If it was mine i would scrap it.
Wouldn't the proper hydro be more than the pressure it would run at? I also work in a refinery and the hydro's are always more than the service the vessel would see on a normal operating run. Plus they always have someone to sign off on everything. I would think if he fixes the tank and something happens down the road a few years he would be liable or injured. Just my .02...Bob

Comment

I have fixed a few compressor tanks in my life time and I have never seen a failure where the tank exploded like a granaede.

Every time I have seen a failure it was nothing more than a split and the air bled out.

I also have a few friends in the portable welding industry and none of them have seen a tank blow into peices like a bomb that many of you suggest.

Keep in mind I'm talking about compressor tanks, usually 2'-3' in dia. max.

I could see a large dia. tank having a big split that if you were standing near the split when it occurres that you could get hit by the splitting metal as it flares out.

Also keep in mind that I am talking about a air compressor tank that typically holds not more than 175 Psi.
Not a 3,000 psi tank.

I would like to hear some feed back as to what the failures were on the air compressor tank failures that you vetrans have seen.
Did the tank slightly split open and the air leak out as I have always seen or did the tank break into peices throwing schrapnal.

Comment

Most common failures I have seen would probably be pinholes from rust, followed by fatigue cracks near mounts. Neither is worth repair, as both indicate that the tank is near the end of its life and problems will recur, unless the tank is something really special (odd size, part of a machine and it can't be replaced, etc)

I have fixed a few compressor tanks in my life time and I have never seen a failure where the tank exploded like a granaede.
.
.
.
I would like to hear some feed back as to what the failures were on the air compressor tank failures that you vetrans have seen.
Did the tank slightly split open and the air leak out as I have always seen or did the tank break into peices throwing schrapnal.

Comment

My dad was a racecar driver when i was a kid. He always carried one of those pancake air tanks on the trailer. One day he filled it up and stopped to BS with a guy and sat the tank down. I went on ahead to the house, i was about 15. A few minutes later he picked up the tank and it blew removing flesh from his right leg. The tank had blown open from top to bottom and looked like a big pillow. He spent the next week in the hospital. That had to hurt...Bob

Comment

I was taught on the procedure for welding pressure vessels, so if you havent been taught this, dont attempt it. If you were going to attempt it, and theirs nothing we can do to stop you, go ahead, and die test the cracked area, and see where the crack start and ends, then if the crack is a perfect straight line, drill out the start and stop of the hairline crack with a 1/8" drill bit, to stop the crack from spreading when you weld it. Then carefully bevel out the crack on both sides of the crack, leaving a 3/32" to 1/8" gap in the middle with the same sized land. Meaning a 3/32' gap, should get a 3/32" land, etc. Then either 6010 the root in or tig the root in. Make sure you ask the manufacturer what grade metal they used for the tank, not letting them know what your doing, otherwise they wont give you the info. Run a root pass, hot pass, and cap, and if done correctly, should leave the weld slightly sticking above the base metal. Then die test again, then hydro. This is a very delicate procedure, and not to be done by the hobby welder, by any means. I would get another tank, but I wanted you to know the correct way of doing it, if you did try to, just so that you wouldnt get hurt or killed.

Comment

If there are fatigue cracks you can see with the naked eye, imagine how many lurk under the surface that can only be detected by magnafluxing.

Sort of like the "see one cockroach, how many are inside the wall" problem.

Now imagine a crack leading in the direction around the "waist" of the tank. Let's say the tank is 30" in diameter.

Force=pressure X area. The cross sectional area is pi*15^2= 707sq in. At 120psi, there is a force of almost 85 thousand pounds trying to blow the tank in half across this section.

If that sounds like a lot, look at the stress on a lengthwise crack...
If the tank is 5' long, the cross sectional area is about 60x30=1800sq in (neglecting domed top/bottom). At 120psi, the force trying to rip the tank apart is 216 thousand pounds.

Yes, when these things fail, it's most commonly a crack that opens slowly enough to let the air out. But it's that failure mode where the crack seeds on an existing fatigue failure plane that allows them to let go suddenly, and with devastating results. This is not a lottery anybody wants to win!

Notice that when they go destructively, they blow out from the side along a longitudinal split (as in the locomotive picture), due to the fact that this plane carries the higher strain.

80% of failures are from 20% of causesNever compromise your principles today in the name of furthering them in the future."All I ever wanted was an honest week's pay for an honest day's work." -Sgt. Bilko"We are generally better persuaded by reasons we discover ourselves than by those given to us by others." -Pascal"Since we cannot know all that there is to be known about anything, we ought to know a little about everything." -Pascal